Field of the Invention
This invention relates to a MCAV (modified constant angular velocity) type information recording/reproducing apparatus, and relates to a method for accessing a desired track on a disk with a head.
An optical disk is provided with a plurality of concentric or spiral tracks. For permitting recording of variable length data and also for accessing tracks at a high speed, each track is divided into a plurality of sectors and recording or reproduction is effected by a sector unit. In order to record the information, a laser beam emitted from a semiconductor laser is condensed on a recording layer on the disk to form a hole in the recording layer or change the reflectivity or transmittance of the layer so as to form a recording pit.
Such information recording apparatus are roughly classified into two types, i.e. , a CLV ( constant linear velocity) type, in which recording pits as noted above are recorded on a disk at a constant linear velocity, and a CAV (constant angular velocity) system, in which recording pits are recorded at a constant angular velocity. In the CAV type, because the pits are recorded at a constant angular velocity, the density of recorded data per unit length is reduced as one goes toward the inner periphery of disk. Therefore, this type is inferior in the aspect of the effective utility of the storage capacity. In the CLV type, the sectors are not radially arranged, and also the rotational number of the disk varies with the position on the disk. Therefore, data can not be reproduced until a target value is reached by the rotational number, at the time of accessing, resulting in a prolonged access time.
Then, there is proposed a MCAV type which features both the advantages of capability of ready accessing in the CAV type and high recorded data density of the CLV type, as disclosed, for instance, in Japanese Patent Kokai 59-167874. In this type, the disk area is divided, in the radial direction, into a plurality of concentric circular zones. The angular velocity of the disk is held constant when the head is on the same zone, and is reduced as the head goes toward the outer zones for recording or reproduction. Another well-known MCAV system, which can provide the same effect, the rotational number of disk is held constant, while the recording/reproducing frequency is increased as one goes toward outer zones for recording or reproduction.
In the meantime, in an information recording/reproducing apparatus using an optical disk as noted above, a track retrieving mechanism is provided for obtaining recording or reproduction with respect to any sector. Usually, such track retrieval is effected as follows. First, index data (including a track address or the like) of the track being presently scanned by a light beam is read out. An optical head is then shifted toward a desired track according to the difference between the address of the desired track and the read-out track address. The address of the destination address is also read out, and if it is not identical with the desired track address, the head is shifted again, or the light beam is kicked in a track-crossing direction with a lens actuator provided in the head, and then the destination track address is confirmed. The above sequence of operations is repeated until the identity of the track address is obtained.
Where the MCAV type is applied to the information recording/reproducing apparatus as noted above, however, a deficiency is rated. More specifically, when the apparatus receives a command to access a desired track, it first causes rotation of the disk at the angular velocity for the zone, to which the desired track belongs. Then, the optical head is shifted, and then the reading of index data is started as above mentioned. A deficiency is encountered at this time if the destination track belongs to a zone different from the desired track. More specifically, in this case no index data can be read out because of the difference of the angular velocity for read-out, thus resulting in a dead lock of the system, i.e., disability to proceed to the next operation. This problem arises as well with a MCAV type, in which the recording/reproducing frequency is varied for each zone.